1. Field of the Invention
The present invention relates to an image forming apparatus and control method therefor and, more particularly, to an image forming apparatus which is suitable for a copying machine, laser beam printer, and facsimile and designed to irradiate an electrostatic medium with a modulated laser beam from a laser beam source to form, on its surface, image information which contains, e.g., electrostatic latent images, and a control method therefor.
2. Description of the Related Art
Conventionally, for image formation, such an image forming apparatus causes a motor to rotate a polygon mirror and detects the scanning position of a laser beam reflected by that polygon mirror to determine the scanning start position in the main scanning direction. That is, to detect the scanning position of the laser beam, a laser beam detecting sensor (to be referred to as a BD sensor hereinafter) is disposed in a non-image area. The BD sensor is irradiated with a laser beam to cause output of a laser beam detection signal (to be referred to as a BD signal hereinafter). The apparatus determines the image write start position on the basis of the resultant BD signal, thus forming an image. A laser beam required to obtain a BD signal during image formation is emitted in stable cycle time because the polygon mirror stably rotates during image formation. Hence, it is a common practice to emit a laser beam in a non-image area at a stage preceding the cycle time during which a BD signal is expected to be obtained.
As a deflecting means for deflecting a light beam from a light source, a galvano mirror which swings a deflective reflection surface is proposed. In recent years, there is also proposed a so-called micromirror which executes sine wave vibration by a resonant structure using a micromachining technology. These proposals are expected to miniaturize optical scanning devices and greatly reduce banding due to vibration, temperature rise, noise, and power consumption.
Moreover, in optical scanning for executing multibeam type reciprocal scanning, Japanese Patent Laid-Open No. 2002-311358 makes it possible to eliminate overlaps of scanning lines and reduce variation of the image height between the scanning line intervals on the surface of a photosensitive member.
Unfortunately, laser reciprocal scanning using a movable mirror raises the following problem. In general, a scanning line written on the electrostatic surface in the main scanning direction upon rotation of the photosensitive member inclines in the sub-scanning direction, though its amount is very small. The rotation speed of the photosensitive member is much lower than the main scanning speed, so the inclination of the scanning line becomes very small. However, when reciprocal scanning is done, the inclination of a scanning line reverses in every scanning. This means that scanning lines are written in zigzags on the surface of the photosensitive member. That is, variation of the image height between the scanning line intervals occurs, in which the interval of a scanning line written on the surface of the photosensitive member changes depending on the image height at a light spot, resulting in an uneven density or degradation in resolution.